This invention relates generally to marine propellers and, more particularly, to an improved marine propeller which includes a hybrid blade configuration providing improved performance at a design point between fully subcavitating and fully supercavitating flow conditions.
Cavitation is an operational characteristic of marine propellers and results when marine propeller blades are rotated at a sufficiently high speed and loading to develop very low pressures along the curved suction side or back of each blade. When the pressures are sufficiently small, a vacuum tends to develop in the low pressure area. The result is that water flowing along the blade back is unable to follow the exact contour of the blade section creating an opening or cavity along the blade back. When such cavities are fully developed in the chord-wise direction and extend beyond the trailing edge of the blade sections, they are known as supercavities and the blade section is operating in supercavitation. Cavitation can only occur in a liquid such as water but not in a gas such as air. The pressure within the cavitation cavities is generally very near the vapor pressure of the liquid.
Ventilation is another operational characteristic of marine propellers in which the cavity pressure is atmospheric. Here the rotating blades either pierce the surface of the water or come so close to the surface that the air is drawn downwardly through the blade tip vortices. This allows atmospheric air to reach the blade through the water opening.
When a marine propeller blade supercavitates below the water surface, the blade is enveloped in a gas cavity containing water vapor at a defined vapor pressure. However, when a supercavitating marine propeller blade operates while ventilating at or near the surface, the gas cavity is at atmospheric pressure.
FIG. 36 in the drawings illustrates the typical operating regimes or flow regions for marine propellers. Conventional marine propeller blade design procedures have been applied to blades which operate totally in the noncavitating region, i.e., low speed at a given thrust, or totally in the fully cavitating region, i.e., high speed at a given thrust, either with or without ventilation. While either design approach or procedure results in propeller designs which operate efficiently at their respective supercavitating or noncavitating design points, such design procedures would suffer from significant inefficiencies when applied to applications where the design point fell within the partially cavitating region of FIG. 36.
In U.S. Pat. No. 4,293,280, a propeller concept is disclosed which attempts to overcome certain problems previously experienced with marine propellers. The concept of U.S. Pat. No. 4,293,280 discloses propeller blades capable of efficient operation at intermediate speed ranges where partial cavitation conditions exist as shown in FIG. 36. Each blade in the propeller of U.S. Pat. No. 4,293,280 includes a radially outer portion with a different blade shape than the radially inner portion so that the outer blade portion has a blunt trailing and a tapered leading edge and a higher blade angle of attack than the inner blade portion. The result is a marine propeller which, in concept, will operate efficiently in coexisting supercavitating and subcavitating flow regions such as the partially cavitating region of FIG. 36.
However, the marine propeller of U.S. Pat. No. 4,293,280 fails to address many practical problems encountered both at the design point and at off design conditions.